A zeolite may be described generally as a crystalline, three dimensional, stable structure enclosing cavities of molecular dimensions.
Zeolites can be natural or synthetic in origin. Naturally occurring zeolites include gmelinite, chabazite, dachiardite, clinoptilolite, faujasite, heulandite, analcite, levynite, erionite, sodalite, cancrinite, mepheline, lazurite, scolecite, natrolite, offretite, mesolite, mordenite, brewsterite and ferrierite. Synthetic zeolites include the zeolites A,B,E,F,H,J,L,Q,T,W,X,Y,Z,N-A, beta, omega, rho, the EU types, the Fu types, the Nu types, the ZK types, the ZSM types, the ALPO. types, the SAPO types, the pentasil types, the LZ series, and other similar materials.
These crystalline zeolitic materials contain well-defined microporous systems of channels and cages. Because the dimensions of these pores are frequently such as to permit adsorption of molecules up to a certain size, with exclusion of larger molecules, zeolites are sometimes referred to as molecular sieves.
Most zeolites are based on aluminosilicate frameworks, the aluminum and silicon atoms being tetrahedrally coordinated by oxygen atoms.
More particularly, these aoluminosilicates are three dimensional networks of SiO.sub.4 and AlO.sub.4 tetrahedra which are linked by sharing of oxygen atoms to give an overall atomic ratio (Si+Al)/O=2. Each AlO.sub.4 unit introduces a single negative charge which must be balanced by an appropriate cation content. When the charge on AlO.sub.4 is balanced by hydrogens or by polyvalent cations such as Ca.sup.2+, Mg.sup.2+ and particularly by rare earth cations, zeolites possess acidic properties and can function as acid catalysts for various types of hydrocarbon conversion.
In some zeolites, atoms of other elements, such as boron, germanium, chromium, iron, phosphorous and gallium, are present in the framework of the crystalline structure. They may be present either in the original zeolite or be introduced into it.
The terms "zeolite" and "zeolitic structure" used hereinafter include similar materials in which atoms of such other elements are present in the framework. Further, they include materials such as pillared interlayered clays ("PILCS"), which have many of the catalytically valuable characteristics of the aluminosilicate zeolites. We also include all modifications to the above materials, whether obtained by ion-exchange, impregnation, hydrothermal or chemical treatments, as later described.
The acid properties of zeolites have been widely utilised in the production of gasoline by the cracking of oil fractions, particularly of gas oil, using FCC technology.
The initial activation of pure hydrocarbons in general by zeolites is still a matter of discussion and both ionic and radical intermediates and also dehydrogenation processes have been proposed in the initial activation. However, the typical product distributions at higher conversion of hydrocarbons over strongly acid zeolites are consistent with carbocation chemistry involving among other reactions typical 8-scission, alkylation, cyclisation and bimolecular hydrogen-transfer processes. The optimal balance of these processes can lead to optimal amounts and quality of gasoline.
Typically, in FCC technology, wide-pore zeolites having the faujasitic structure (X or Y zeolites) have been employed but improvements have been achieved by including in the catalyst composition other additional zeolites, including medium pore zeolites of the ZSM-5 family, or by adding molecular sieves based on AlO.sub.4, PO.sub.4 and SiO.sub.4 tetrahedra (e.g. SAPO-37). Various modifications of zeolite Y, including ion-exchange, hydrothermal treatment and chemical treatment have provided improved catalysts. Hydrothermal treatments and some chemical treatments result in replacement of some of the framework AlO.sub.4 by SiO.sub.4 tetrahedra.
A chemical treatment, applicable to a wide range of zeolites, for replacing some of the framework AlO.sub.4 by SiO.sub.4 tetrahedra is described in EP-A-0082211. In this process a zeolite, especially zeolite Y and preferably in ammonium form, is reacted with an aqueous solution of a fluorosilicate salt, especially ammonium silicon hexafluoride. This results in a range of faujasitic structures known as the LZ series.
Such processes can modify the strength and number of the catalytically active acid sites and are associated with a decrease in the unit cell parameter of the zeolite Y as described in more detail below. The activity and selectivity of FCC catalysts can be correlated to the unit cell parameter of the zeolite; see Pine, LA and et al, J. Catalysis (1984), 85, 466. A cell parameter less than that of a typical Y zeolite has been found to be advantageous.
The incorporation of gallium into various zeolite catalysts is known to change their behaviour.
Thus, U.S. Pat. No. 4,377,504 and U.S. Pat. No. 4,415,440 describe the impregnation of a zeolite catalyst (of unspecified type), contaminated with metals such as vanadium and iron, with gallium. This restores the selectivity of the contaminated catalyst. In such catalysts the gallium is merely present as a component of a catalyst mixture.
EP-A-0147111 teaches impregnation and/or ion exchange for introduction into zeolite catalysts suitable for cracking of ethane rich low hydrocarbon foodstocks.
EP-A-0292030 describes a process for preparing a modified zeolite Y for hydrocracking in which the zeolite is treated with a solution of a gallium salt to effect ion exchange and thereafter calcined to provide a product having a low unit cell size of from 24.21-24.6 .ANG.. Such products give improved catalytic activity in hydroconversion processes.
Ion exchange processes for introducing gallium ions into zeolites are also described in EP-A-0024930 and EP-A-0258726, which latter describes gallium ion exchange of zeolites for providing an FCC catalyst giving an increased aromatic content.
In such ion exchange processes, the gallium is present as gallium ions which may be attached to the zeolite structure.
U.S. Pat. No. 4,803,060 and JP-A-62-179593 describe the primary, i.e. direct, synthesis of zeolites in which the gallium forms part of the crystal framework structure. The zeolites are prepared from solutions capable of reacting so as to provide a crystalline zeolite structure containing gallia and silica alone (U.S. Pat. No. 4,803,060) or gallia, alumina and silica (JP-A-62-179593). The zeolites of U.S. Pat. No. 4,803,060 are used for hydrocracking, while those of JP-A-62-179563 are used for cracking hydrocarbons in general to increase the aromatic content, the cracking ability of the zeolites being tested with reference mainly to n-hexane.
U.S. Pat. No. 4,524,140 and U.S. Pat. No. 4,620,921 describe the secondary synthesis of zeolites in which boron or iron present in the crystal framework structure of a zeolite is replaced by gallium. This substitution is achieved by a hydrothermal activation technique using liquid water at high temperature and pressure in the presence of gallium chloride. The product had an improved c-value for cracking n-hexane.
EP-A-0134849 describes the reaction of high silica zeolites with various volatile metallic compounds having a radius ratio below 0.6, including chloride vapour. The zeolite is calcined before reaction with the volatile compound to create vacancies in the lattice into which the metal may be introduced. No structural data are given to substantiate the presence of the metal in the framework structure.
EP-A-0187496 describes the synthesis of zeolites containing gallium in which a high silica zeolite (SiO.sub.2 /Al.sub.2 O.sub.3 =26,000) is treated with an aqueous solution containing gallium at a pH of at least 7. Again, no structural data are given, so there is no confirmation that the gallium enters the framework structure.
Likewise U.S. Pat. No. 4,891,463 describes the synthesis of ZSM 5 zeolites containing gallium in which high silica ZSM 5 zeolites are treated with an aqueous alkali solution containing gallium. It is claimed that at least some of the gallium is present in the framework structure, though no mention is made as to whether the gallium replaces silicon or aluminium.
Each of the above processes provides a zeolite catalyst in which gallium is present in some form or other, i.e. either as gallia in admixture with the zeolite, as gallium ions or as gallium within the crystal framework structure.
However, we find that in all of the above structures either the gallium is not present in a form in which it has maximum effect, especially in FCC technology, or it is necessary to use very large amounts of gallium in the system to achieve the improved effect.
In particular, when incorporating gallium into a faujastic structure by direct synthesis a particularly large amount of gallium is present in the starting reaction medium and yet only a small amount is finally incorporated into the crystal framework structure.